Flow-forming is a cold deformation process to form dimensionally precise and rotationally symmetrical parts. Strain hardening effect of the flow forming process, and possibility of producing cylindrical part are the advantages especially for aerospace industry. The purpose of this study is to investigate the effect of initial microstructure of an AISI 4140 steel on the microstructure and mechanical properties after being flow formed by 70% as the reduction ratio in the thickness direction. In this context, as-received steel was heat treated to standard quenched and tempered condition, and an additional annealing was also performed. Before and after the flow forming process, the microstructure was examined, hardness and tensile tests were conducted. The results revealed that the additional annealing was beneficial to obtain a crack free material after the flow forming process of a heat treated material.